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Move IgnorePaintTimingScope from paint_timing_detector.h into dedicated
.h and .cc files under renderer/platform, so that PaintController can
check it before recording first paint status.

This fixes several wpt tests for FCP about opacity in
CompositeAfterPaint which always paint opacity:0 contents. This doesn't
fix (and probably we'll never fix) missing FCP when opacity:0 becomes
non-zero, because we don't repaint.

Change-Id: If1b393f5c1440fada11806a7ac071b5700843ac5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145134
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758350}